Reed valves in kart engines, fundamental for performance and reliability

REED VALVES

TKART Staff

01 March 2017

They are the most widely used method of regulating the mixture intake flow in 2-stroke engines. Geometry and materials, as well as the position of these components, are fundamental aspects for the performance and reliability of kart engines

WHAT IS IT?

HOW DOES IT WORK?

IN PRACTICE

2T engines do not have intake and exhaust valves, unlike 4T. Instead, they use different systems that regulate the mixture flow from the carburettor to the engine. One solution is called “reed valve intake”, another “piston port”, and a third “rotating valve”. Starting from the latter, one could say that the system certainly simplifies the structure of the engine, but at the same time does not offer the performance of the first two mechanisms. Nowadays, with racing engines mainly designed for elasticity of use, rotating valves have been put to one side a little in favour of the other two systems. The “piston port” is compulsory in many junior class regulations.
However, the most widely used is now the “reed valve” system, which has the ability to adapt to the various engine speeds more effectively than the rotating valve.

Since the reed valve works with a frequency of opening and closing that, in theory, is equal to the number of engine revolutions, it's clear that this component must be extremely light, in order to minimise inertia due to weight and follow engine revs more effectively; durable, so that it can flex quickly countless times without breaking; elastic, so that it quickly returns to the default position (closed) after each engine revolution. The materials that are best suited to these mechanical features are composites such as glass fibre and, better still, carbon fibre. Consider that the modulus of elasticity, i.e. the value that indicates the ability of a material to return to its original shape once elastically deformed, of carbon fibre is up to 3 times that of steel, while specific weight is less than a quarter and specific resistance exceeds 10 times that of steel.

ON 2-STROKE ENGINES, THE INTAKE CAN BE REGULATED BY PISTON PORT, REED VALVE OR ROTATING VALVE SYSTEMS. THE LATTER NOW HAVING ALMOST DISAPPEARED

The components of reed valve pack

WHAT IS IT?

HOW DOES IT WORK?

IN PRACTICE

REED VALVE DISTRIBUTION

THE EFFECT OF THE EXHAUST RESONANCE ON REED VALVE OPENING

The waves of pressure generated by the exhaust ports reach the reed valve pack through the combustion. In this way, even before the depression in the carter pump occurs, the depression waves reach the reed valves and lift them

RESONANCE OF THE REED VALVE

One of the limits of reed valves is that they sometimes “resonate” at high engine speeds, flexing in the opposite direction to that of the opening or closing, and thereby reducing the open section or even the partial closure of the passage.

Reed valves may have different forms, but in general in kart engines they are made in pairs consisting of two petals shaped like an isosceles trapezium with rounded corners (to avoid breakage and turbulence affecting the mixture flow). The reed valves can then have holes (reduced and well finished to limit the weakening of the structure) at the base that attaches to the reed valve pack, in areas in which mixture does not flow: